[CodeGen] Fixing inconsistent ABI mangling of vlaues in SelectionDAGBuilder

[CodeGen] Fixing inconsistent ABI mangling of vlaues in SelectionDAGBuilder

SelectionDAGBuilder was inconsistently mangling values based on ABI
Calling Conventions when getting them through copyFromRegs in
SelectionDAGBuilder, causing duplicate value type convertions for
function arguments. The checking for the mangling requirement was based
on the value's originating instruction and was performed outside of, and
inspite of, the regular Calling Convention Lowering.

The issue could be observed in a scenario such as:

```
%arg1 = load half, half* %const, align 2
%arg2 = call fastcc half @someFunc()
call fastcc void @otherFunc(half %arg1, half %arg2)
; Here, %arg2 was incorrectly mangled twice, as the CallConv data from
; the call to @someFunc() was taken into consideration for the check
; when getting the value for processing the call to @otherFunc(...),
; after the proper convertion had taken place when lowering the return
; value of the first call.
```

This patch fixes the issue by disregarding the Calling Convention
information for such copyFromRegs, making sure the ABI mangling is
properly contanined in the Calling Convention Lowering.

This fixes Bugzilla #47454.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D87844